Reading the Language of Firefly Flashes

Flashing flirted its way up the firefly family tree.These beetles’ evolutionary history shows a strange metamorphosis unfolding. Firefly eyes grow bigger, more bug-like, as the insects’ light organs enlarge. Their antennae, used like a nose to follow pheromones, shrink into stubs. The more important bioluminescent courtship signaling became throughout their history, the more the trappings of invisible communication faded out.

When Marc Branham, a professor of entomology at the University of Florida, began researching fireflies, he assumed such a beloved animal would be a textbook case in entomology. He was shocked to learn how little scientists knew about the common insect.

What researchers did know was that each species of bioluminescent adult firefly has its own flash fingerprint. Males fly through the air and search for females with a species-specific light display. Some flash only once. Some emit “flash trains” of up to nine carefully timed pulses. Others fly in specific aerial patterns, briefly dipping before sharply ascending and forming a “J” of light. A few even shake their abdomens from side to side and appear to be twinkling. “So if you’re looking over a field,” says Branham, “You can pretty accurately tell how many species are in that area.”

What scientists did not fully understand was how these signaling systems radiated into this diverse ensemble. Branham suspected it might have to do with female preference. As males dart through the air, performing luminous gymnastics for sex, females sit, often immobile, on vegetation, waiting to start a flash dialogue with the right male. “Females are pretty choosy about who they flash back to,” Branham explains. And they should be: females have a great investment in the offspring since the eggs are more expensive than sperm to produce.

To test his theory of female choice, Branham resorted to the unnatural. He created a computer-driven robotic firefly that could mimic the flash pattern of each species he studied. Branham could also tweak parameters like light color, intensity, and timing and enter the field to gauge how mates reacted. When he made the artificial firefly pulse at strobe-light speeds, beyond those possible in nature, females sparked like live wires.